At the Limit of Interfacial Sharpness in Nanowire Axial Heterostructures

As semiconductor devices approach dimensions at the atomic scale, controlling the compositional grading across heterointerfaces becomes paramount. Particularly in nanowire axial heterostructures, which are promising for a broad spectrum of nanotechnology applications, the achievement of sharp heterointerfaces has been challenging owing to peculiarities of the commonly used vapor–liquid–solid growth mode. Here, the grading of Al across GaAs/AlxGa1–xAs/GaAs heterostructures in self-catalyzed nanowires is studied, aiming at finding the limits of the interfacial sharpness for this technologically versatile material system. A pulsed growth mode ensures precise control of the growth mechanisms even at low temperatures, while a semiempirical thermodynamic model is derived to fit the experimental Al-content profiles and quantitatively describe the dependences of the interfacial sharpness on the growth temperature, the nanowire radius, and the Al content. Finally, symmetrical Al profiles with interfacial widths of 2–3 atomic planes, at the limit of the measurement accuracy, are obtained, outperforming even equivalent thin-film heterostructures. The proposed method enables the development of advanced heterostructure schemes for a more effective utilization of the nanowire platform; moreover, it is considered expandable to other material systems and nanostructure types.


Figure S1 :
Figure S1: Measurement of diameter variations along a nanowire.(a) Geometry of a hexagonal nanowire.Lengths  and ℎ represent the side and the projected thickness along 〈11 ̅ 0〉 direction, respectively.(b) HRSTEM image of a nanowire.The inset shows the intensity profile obtained from the rectangular area. can be directly measured from the profile width, whereas ℎ = √3.(c) Representative diameter measurements at the insertion positions along a nanowire (samples A, D, F).The corresponding maximum diameter variations Dmax-Dmin (typically less than 0.5 nm) are listed.

Figure S2 .
Figure S2.Statistical analysis for the evaluation of the reproducibility of our growth method.The degree of reproducibility can be evaluated by comparing measured Al profiles in

Figure S3 :
Figure S3: Error bars in Al-content measurements.The Al profile around the first AlxGa1-xAs insertion (Al pulse of 8 s and 0.05 ML/s) in Fig. 1e including error bars.Typically, the error in x is in the range of ± 0.05.

Figure S4 :
Figure S4: Demonstration of the contribution of electron cross-scattering into abutting MLs.Two AlxGa1-xAs profiles are shown for 40-nm-thick nanowires (  = 20 nm).Black squares connected by a dashed line represent a profile containing the real Al content present in the MLs of the simulated supercell, chosen manually as follows: from left to right x = 1.0, 1.0, 1.0, 0.6, 0.6.The red profile is the output of the HAADF image simulation including the influence of electron cross-scattering.Red arrows highlight two extreme cases in which cross-scattering yields false Al contents in Al-containing MLs (right arrow) and even creates a completely false Al signal in an Al-free ML (left arrow).

Figure S5 :
Figure S5: Fit parameter  plotted as a function of nanowire radius   .Circles, squares, and triangles represent insertions grown at 550, 450, and 350 °C, respectively.The color gradient is a scale for the total Al content .The dashed line is a guide to the eye highlighting  = 1.

Figure S6 :
Figure S6: Comparisons of the interface parameters  and  to both the total Al content  and the nanowire radius   demonstrating the behavior of the profile slopes.(a) GA interface parameter  plotted as a function of the total Al content  with a color gradient scale for   .(b) As the plot in (a), but here for the AG interface parameter .Circles, squares, and triangles represent insertions grown at 550, 450, and 350 °C, respectively.

Figure S7 :
Figure S7: Tablecontainingthe growth conditions and parameters for all samples in this work.Gray-and yellow-shaded rows correspond to growth steps -2 and -3, respectively.Shown are the growth temperature (  ), the nanowire radius (  ), and the flux and duration of Ga (  ,   ), Al (  ,   ), and As4 (  ,   ) pulses. −1 corresponds to the duration of the first As4 pulse after Al.

Figure S10 .
Figure S10.Comparison of compositional analysis by HAADF-STEM imaging and EDXS.(a) High-resolution HAADF-STEM image.(b) EDXS compositional map: red for Al and blue for Ga.(c) Extracted Al-content profiles from the framed areas with green rectangles in (a) and (b).Both methods resolved the rectangular shape of the insertion and resulted in similar plateau values for the Al content and similar insertion widths.In contrast to HAADF-STEM, though, EDXS failed to resolve the local (1 ML-thick) peak of Al content at the beginning of the insertion owing to the lack of atomic resolution.The block arrows in all panels indicate the growth direction.